The basic objectives of the proposed research are to study renal tubular function by in vitro microperfusion techniques. During the forthcoming fiscal year the specific areas which we wish to examine are: 1) Examine the possibility that two functionally different populations of proximal tubules exist in the mammalian kidney. 2) Describe the anatomical characteristics of the thin descending and ascending limbs by electron microscopic techniques. 3) Study the differences between functions of the distal convoluted tubule as contrasted to the cortical collecting duct.